The invention relates to a method for estimating an impulse response in a digital radio system, in which the signals to be transmitted comprise bursts formed of symbols, the bursts comprising a known training sequence, the method comprising sampling from a received signal, measuring of a DC offset in the samples and correlating the signal with the known training sequence.
In a typical cellular radio environment the signals between a base station and a subscriber terminal propagate on several routes between a transmitter and a receiver. This multipath propagation is mainly caused by signal reflections from surrounding surfaces. Signals travelling on different routes arrive at the receiver at different times because of a different propagation delay. This holds true for both transmission directions. The multipath propagation of a signal can be monitored in a receiver by measuring the impulse response of the received signal, signals arriving at different times being visible as peaks proportional to their signal strength. FIG. 1 illustrates a measured impulse response by way of example. Time is on horizontal axis 100 and the strength of the received signal is on vertical axis 102. Peaks 104, 106, 108 of the curve indicate the strongest multipath propagated components of the received signal.
In prior art solutions the impulse response is estimated using a known training sequence added to the burst. In known solutions, e.g. in the GSM system, the impulse response is estimated by cross-correlating the received samples with a known training sequence. FIG. 2 illustrates one example of a prior art solution. Received signal 200, converted into an intermediate frequency or to a baseband, is transmitted to sampling means 202 in which samples taken 204 are transmitted to memory means 206 from which digital signal processor 210 reads samples 208 and processes them. Samples 204 comprise a DC offset which is removed before demodulation and impulse response calculation. This causes delay in signal processing. In the signal processor the DC offset is removed, then the signal is demodulated and the impulse response can be calculated by correlating signal with the training sequence. The correlation comprises adding and subtracting operations, and the digital signal processor unefficiently performs said operations compared with the solutions built out of special components. Access burst calculation in particular is a substantial load for the signal processor.
An object of the present invention is to implement a method and a receiver in which a signal processor load can be reduced and with which an impulse response can be rapidly and efficiently calculated.
This is achieved with the method set forth in the preamble characterized in that the method comprises impulse response calculation by correlating the samples taken with a training sequence before the DC offset is removed and the removal of the DC offset effect from the calculated impulse response.
The invention also relates to a receiver in a digital radio system where a received signal comprises bursts formed of symbols, the bursts comprising a known training sequence, the receiver comprising means for sampling the received signal, and means for measuring a DC offset in the samples. The receiver of the invention is characterized in that the receiver comprises means for calculating an impulse response by correlating the samples taken with the training sequence before the DC offset is removed, and means for removing the DC offset effect from the calculated impulse response.
The solution of the invention has several advantages. The solution of the invention is easy to implement. For example in the GSM system the impulse response calculation of an access burst is the heaviest calculation operation of a frequency equalizer in a GSM base station. With the solution of the invention the calculation can be implemented twice as easy in comparison with current solutions. Furthermore, the solution of the invention allows impulse response calculation in connection with sampling, thus speeding up the calculation. This was not previously possible due to the error caused by the DC offset.